462 CONSIDERATION OF CARBON COMPOUNDS. 



into two large classes, each one embodying all the derivatives of one 

 of the two hydrocarbons named. The derivatives of methane are 

 often termed fatty compounds, those of benzene aromatic compounds. 

 Methane derivatives have representatives in each one of the above 

 ten groups : benzene derivatives are missing in a few. As far as 

 practicable, the two classes will be considered separately, because 

 the properties of fatty and aromatic compounds diifer so widely, in 

 some respects, that this method of studying the nature of carbon 

 compounds is to be preferred. 



42. HYDROCARBONS AND THEIR HALOGEN DERIVATIVES. 



Occurrence in nature. Hydrocarbons are seldom derived from 

 animal sources, being more frequently products of vegetable life; 

 thus, the various essential oils (oil of turpentine and others) of the 

 composition C 10 H 16 or C 20 H 32 are frequently found in plants. 



Other hydrocarbons are found in nature as products of the decom- 

 position of organic matter. Thus methane, CH 4 , is generally formed 

 during the decay of organic matter in the presence of moisture ; the 

 higher members of the methane series are found in crude coal-oil. 



Formation of hydrocarbons. It is difficult to combine the two 

 elements carbon and hydrogen directly; as an instance of such direct 

 combination may be mentioned acetylene, C 2 H 2 , which is formed 

 when electric sparks pass between electrodes of carbon in an atmos- 

 phere of hydrogen. 



Many hydrocarbons are obtained by destructive distillation of 

 organic matter, and their nature depends on the composition of the 

 material used and upon the degree of heat applied for the decompo- 

 sition. Hydrocarbons may also be obtained by the decomposition 

 (other than destructive distillation) of numerous organic bodies, such 

 as alcohols, acids, amines, etc., and from derivatives of these sub- 

 stances. 



The hydrocarbons found in nature are generally separated from 

 other matter, as well as from each other, by the process known as 

 fractional distillation. As the boiling-points of the various compounds 

 differ more or less, they may be separated by carefully distilling off 

 the compounds of lower boiling-points, while noting the temperature 

 of the vapors above the boiling liquid by means of an inserted ther- 

 mometer, and changing the receiver every time an increase of the 

 boiling-point is noticed. This separation of volatile liquids, known 

 as fractional distillation, is, however, not absolutely complete, because 



